Lithography is recognized as one of the most widely used methods of printing. In its usual commercial or industrial application it is known as offset lithography because the inked image is first printed on a rubber cylinder, which then offsets or transfers the image to paper or other material. In lithography, the matter to be printed is neither raised above nor sunk below the surface of the plate but remains on the surface, or plane.
X-ray lithography, developed in the last twenty years, has become an important aid to the art of lithography. X-ray lithography is now considered to be an effective method for pattern replication down to the submicron range, required by the huge, development of the technology for the production of microelectronic circuits. In this technique, by a beam of X-rays of low angular divergence, a shadow image of the mask pattern is projected onto the wafer which is coated with an X-ray sensitive resist. Accordingly, high demands must be made on the masks concerning the dimensional stability and accuracy of the absorber pattern since submicron structures must be transferred.
In a very recent review (X-ray Lithography, IEEE Spectrum, February 1989), it is stipulated that several requirements still have to be met in order to produce technologically and economically viable X-ray lithography masks. It is also pointed out that the present masks cannot take the service stress. The literature is quite abundant with various patents and reviews concerning the manufacture of masks to be useful for X-ray lithography. The basic approach consists of an X-ray transparent membrane, stretched in a supporting frame, and an X-ray absorbing metal structure applied onto the membrane. Thus, silicon substrates are provided with a highly boron-doped layer, which acts as a stop layer for the membrane preparation. The wafer is then bonded to a glass plate and the silicon membrane is obtained by a selective etching technique. After deposition and patterning of the absorber, metallization is performed. One of the problems with metal films deposited on a substrate is a result of the considerable stresses in the metallized absorber layer which may cause distortion of the desired pattern.
Some improvements to remedy the above problems are described in some recent patents. Thus, according to Japanese Kokai 58/73118 (cf. C.A. 100, 148511), X-ray transmitting support films for X-ray lithography photo-masks are prepared by coating substrates such as silicon wafer with an organic metal complex solution and subsequently calcining the coating at a temperature of about 600.degree. C. It is claimed that the method gives support films with very few defects. After electron-beam lithography treatment of the metal layers, the silicon wafer was removed by photoetching to give a high quality X-ray photomask.
An amorphous hydrogenated boron-nitride or boron carbonnitride film made by plasma chemical vapor deposition is claimed to be useful for an X-ray transparent mask membrane (Japanese Kokai 63/76430 cf. C.A. 109, 139745). It is claimed that peeling of deposits on the electrodes and the susceptor was suppressed and thus contamination of the film with peeling dust could be prevented.
It has been reported (R. E. Acosta et al., Microelectronic Engineering 3, 1985 p. 573-9) that the distortion of X-ray masks is directly related to the stress of the absorber used. This distortion, resulting from the material absorbed, causes a decrease in the mask characteristic pattern and could render the masks unusable if the stress is not minimized. In the case of films deposited on single crystal silicon substrates, the distortion can be determined with the aid of an X-ray diffraction technique. In this case it is necessary to determine the crystalline integrity of the substrate before the deposition of the film to be studied, and to ensure that no damage to the crystal occurs during the process of deposition of the film. For non-crystalline substrates, the curvature of the substrate can be determined either by mechanical or by optical means.
In a very recent patent application (German 3,634,147, cf. C.A. 107, 208862), a lithographic mask structure is described. The mask consists of a ring-like support and a masking layer support containing a luminophor to improve the sensitivity of the resistive material.
In an Abstract by T. Funayama et. al. (J. Vac. Sci. Technol., Vol. 12, No. 6, Nov/Dec. 1975, 1324), a new X-ray lithography mask is described. The mask consists of an aluminum substrate and an Al.sub.2 O.sub.3 film which was grown on said aluminum substrate by anodization. The transparent aluminum membranes of alumina film were made by chemically etching away parts of the aluminum substrate.
In a review by P. A. Sullivan (I.E.E.E. Transactions on Electron Devices, Vol. ED-23, No. 4, April 1976, 412-7) on Determination of Wavelength and Excitation Voltage for X-Ray Lithography, a similar X-ray mask based on aluminum is used, the aluminum oxide being grown on an aluminum substrate having a thickness of 75 microns. The disadvantage of the masks described above, is the fact that the frame which remains after the etching is actually a foil of aluminum metal which is very soft and ductile and therefore suffers from permanent distortions during its use. Moreover, during the distortion of the substrate and support the entire membrane will be fractured.